This commit enables the posting of broadcast messages for admins in admin control center. Moreover "handlers" are renamed to "services" from now on.

Classes:
- services: Refactor class names from Handler to Service, add and implement a more general TemplateService to load and render templates, use TemplateService in EmailService
- AdminController.java: Implement broadcastNotificationForm() and async (non blocking) broadcastNotification() for GET and POST
- PostController.java: Code improvements in deletePost()
- other controllers: Affected by renaming of NotificationService
- BaseNotifiable.java: Refactor render() method to use the new TemplateService to "keep it DRY", improve addTemporaryRecipient() by checking, if a temporary recipient is already added
- Account.java: Implement getAccountListByIdCollection() to get a list of accounts by a list of IDs as String
- Post.java: Add attributes isBroadcastMessage, broadcastPostRecipients and constant BROADCAST, add fetching of broadcast posts in streamForAccount(), implement addRecipient()
- Global.java: Implement nextExecutionInSeconds() and nextExecution() to determine correct schedule time (e.g. 18:00 o'clock) for email service schedule in onStart()

Templates:
- Admin/snippets/navigation.scala.html: Add broadcast form to control center navigation, add i18n
- Admin/admin.scala.html: Add i18n
- Admin/createBroadcastNotification.scala.html: Add template for broadcast post form
- Emails/notifications*.scala.html: Refactor due to TemplateService parameters
- NewNotification/post/broadcast.scala.html: Add template for broadcast notification
snippets/postView.scala.html: Add CSS for broadcast posts

Other:
- messages: Add i18n for broadcast notification and admin center texts
- routes: Add routes for broadcast notification post handling
- htwplus.css: Add CSS for broadcast posts